Move the Moodle README from mooc-grader to the aplus-manual

[markkuriekkinen]

mooc-grader/doc/README_Moodle_frontend.md (https://github.com/apluslms/mooc-grader/blob/master/doc/README_Moodle_frontend.md) could be moved to the aplus-manual into the Astra module. It requires some re-organizing.

In addition, there is a difference in Astra that is not mentioned anywhere. In Astra, exercise max_submissions may be a negative number in which case Astra stores only the N latest submissions instead of all submissions (i.e., -1 means only the one latest submission is stored). There is then no limit on how many times a new submission can be uploaded. A+ does not support negative max submissions values. Zero ( = unlimited) and positive max_submissions limit works the same way in Astra and A+.